The following discussion of the prior art is intended to present the invention in an appropriate technical context and allow its significance to be properly appreciated. Unless clearly indicated to the contrary, however, reference to any prior art in this specification should not be construed as an admission that such art is widely known or forms part of common general knowledge in the field.
Separation devices, such as thickeners, clarifiers and concentrators, are typically used for separating solids from suspensions (typically containing solids suspended in a liquid) and are often found in the mining, mineral processing, food processing, sugar refining, water treatment, sewage treatment, and other such industries. These devices typically comprise a tank in which solids are deposited from a suspension or solution and settle toward the bottom as pulp or sludge to be drawn of from below and recovered. A dilute liquor of lower relative density is thereby displaced toward the top of the tank, for removal via an overflow launder. The suspension to be thickened is initially fed through a feed pipe, conduit or line into a feedwell disposed within the main tank. A rake assembly is conventionally mounted for rotation about a central drive shaft and typically has at least two rake arms having scraper blades to move the settled material inwardly for collection through an underflow outlet.
In its application to mineral processing, separation and extraction, a finely ground ore is suspended as pulp in a suitable liquid medium such as water at a consistency which permits flow, and settlement in quiescent conditions. The pulp is settled from the suspension by a combination of gravity with or without chemical and/or mechanical processes. Initially, reagents, such as coagulant and/or flocculant, can be added into the suspension to improve the settling process. The suspension is then carefully mixed into the separation device, such as a thickener, to facilitate the clumping together of solid particles, eventually forming larger denser “aggregates” of pulp particles that are settled out of suspension.
Known separation devices of this type typically include a thickening tank and a feedwell disposed generally within the tank to contain feed slurry. The feedwell is typically comprised of a single chamber having an inlet for receiving feed slurry, and an outlet for the slurry to descend into the thickening tank. The feedwell is configured to reduce the turbulence of the incoming feed stream, to allow reagents to be mixed into the slurry, and to allow for reaction between reagents and the feed material before the treated slurry is discharged into the thickening tank. Diluted liquid in the thickening tank may also be added to the feed liquid in the feedwell to increase the efficiency of the coagulation and/or flocculation process. The configuration of the feedwell may also promote even distribution of slurry flowing from its outlet into the thickening tank. Where the separation device includes a rake assembly to facilitate the flocculation and separation processes, the rake is typically rotatably mounted on a central drive shaft within the thickening tank and a lifting mechanism is used to vertically adjust the rake assembly to vary its depth in the thickening tank.
One type of feedwell employs one or more planar plates or “shelves” to extend the retention time of the feed slurry in the feedwell, thereby enhancing mixing of the reagents into the slurry and thus the reaction between the reagents and the feed material. Increasing the shelf width renders the shelf effective in enhancing mixing and flocculation. However, a disadvantage of these shelves is that flocculated solids tend to accumulate on the shelf in the form of a sticky “mud”, thus rendering the shelf ineffective or less effective for its intended purpose. This requires the feedwell to be frequently cleaned, which necessitates shutting down the thickener and thus results in loss of production. This problem is exacerbated with shelves of increased width, as this tends to increase the amount of solids that build up on the shelf as well as the probability of this occurring.
Although the presence of one or more shelves assists retention of the feed slurry in the feedwell, there is no control of the flow of the slurry in the feedwell chamber. As a consequence, a substantial portion of the slurry may flow off the shelf or shelves and downwardly out of the feedwell chamber without mixing sufficiently with reagents and/or dilution liquid. Also, the slurry may exit the feedwell at too high a velocity, hindering clumping together of solid particles, and hence hindering or preventing the formation of the desired pulp aggregates.
Another type of feedwell has a deflection cone at the base of the feedwell chamber to define a restricted outlet for the feedwell. The restricted outlet assists with maintaining residence time in the feedwell chamber and enables the treated slurry to exit the feedwell at the desired velocity to encourage the formation of pulp aggregates in the settling process, while the deflection cone provides more even distribution of the treated slurry into the thickening tank. As a feedwell of this type increases in size, the deflection cone must also increase in size to maintain the size of the restricted outlet and hence the required exit velocity and even distribution of the treated slurry. However, at larger sizes, the deflection cone extends further downwardly into the thickening tank, and thus can interfere with the operation of the rake assembly, especially the lifting mechanism for vertically adjusting the height of the rake assembly along its drive shaft.
Increasing the size of the separation device also generally increases the size of the feed slurry flows. Thus, the feed pipe and the associated feed inlet of the feedwell must also increase in size to accommodate the increased volumetric flow rate of the incoming feed slurry. As the feedwell increases in size, the feed inlet tends to occupy a proportionately greater area of the sidewall, and in some cases can effectively require the entire height of the feedwell chamber sidewall. This results in the incoming slurry tending to flow into and out of the feedwell without the necessary residence time in the feedwell chamber, inhibiting optimal mixing of the slurry with reagents and failing to allow the turbulence in the slurry to quiesce, which is required in order to promote optimal settling in the thickening tank. As a consequence, the volumetric capacity of the feedwell is limited by the diameter of the feed inlet, which in turn is limited due to the need to confine the feed inlet generally in an upper portion of the feedwell.
A further type of feedwell has two separate chambers, typically arranged as an upstream chamber and a downstream chamber. It has been found that agglomerated particles tend to accumulate or build-up at the base of the upstream chamber, commonly called “sanding”. As mixing of the feed slurry with reagents and/or dilution liquid tends to occur in the upstream chamber, this sanding phenomenon hinders efficient mixing and reduces the overall residence time of the slurry in the feedwell, and hence settling in the tank.
It is an object of the invention to overcome or ameliorate one or more of the deficiencies of the prior art, or at least to provide a useful alternative.